As is known, a laser beam does not exhibit a homogeneous structure. The beam contains, for example, transient phenomena known as hot spots or speckles that appear and migrate within the beam as a function of time. As the distance between the laser beam and a target increases, other effects such as scintillation and warping or distortion become more and more pronounced and important. It is desirable, therefore, to provide a rapid response time instrument which can measure the fine grain energy distribution within a high energy laser beam, particularly lasers such as the carbon dioxide type operating in the far infrared field. Such an instrument must be capable of measuring parameters in each unit area of a target surface that will permit the determination of average power, average power density, power centroid, laser spot size and shape, and temperature. Furthermore, the target must be able to withstand incident wave energy levels of 5 kilowatts or greater per square centimeter and must be capable of withstanding a useful, extended duty cycle.